Conventionally, as a method for controlling the flow rate of a refrigerant using an electric expansion valve so that the state quantity of a refrigeration cycle, such as temperature, pressure, or the like of the refrigerant is to be a predetermined value, a method has been proposed in which the control gain of PID control stored in a microcomputer in advance is calculated in accordance with indoor temperature, outdoor temperature, and rotation speed of a compressor, and the electric expansion valve is driven by PID control to thereby control the flow rate of the refrigerant (see, for example, Patent Literature 1).
Another method has been proposed in which the control gain and control intervals of PID control stored in advance as a table in a microcomputer are determined in accordance with the number of interconnected indoor units which are in operation and a deviation from a control target value, and an electric expansion valve is driven by PID control to thereby control the flow rate of a refrigerant (see, for example, Patent Literature 2).
In addition, there has been another method proposed in which the control gain and control intervals of PID control are determined in accordance with the opening degree of an electric expansion valve, the compressor operating capacity, and a deviation from a control target value, and the electric expansion valve is driven by PID control to thereby control the flow rate of a refrigerant (see, for example, Patent Literature 3).